1. Field of the Invention
This invention relates generally to the treatment of carbonaceous graphitizable pitches so as to render them particularly useful in the formation of shaped carbon articles and especially carbon fibers.
2. Description of the Prior Art
Carbon artifacts have been made by pyrolyzing a wide variety of organic materials. One carbon artifact of commercial interest today is carbon fiber. Hence, although particular reference is made herein to carbon fiber technology, it will be appreciated that this invention has applicability in areas other than carbon fiber formation.
Referring now in particular to carbon fibers, suffice it to say that the use of carbon fibers in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforcing composite materials, such as their high strength to weight ratios, clearly offset the generally high costs associated with preparing them. It is generally accepted that large-scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many carbonaceous pitches are known to be converted at the early stages of carbonization into a spherical liquid crystal called mesophase. The presence of this ordered mesophase structure prior to carbonization is considered to be a significant determinant of the fundamental properties of the carbon fiber. Unfortunately the rate of mesophase formation from pitches is low. Additionally, carbonaceous pitches containing relatively large amounts of mesophase have relatively high softening points and viscosities making them difficult, if not impossible, to spin into fibers. Also, mesophase formation occurs at relatively high temperatures where incipient coking occurs. The presence of coke prior to spinning is also detrimental.
Significantly, it recently has been discovered that typical graphitizable carbonaceous pitches contain a separable fraction which possesses very important physical and chemical properties including: (1) a softening point and viscosity suitable for spinning; and (2) the ability to be converted in generally less than about 10 minutes and especially in less than about 1 minute when heated to temperatures in the range of about 230.degree. to about 400.degree. C., to an optically anisotropic deformable pitch material containing greater than 75% of a liquid crystal type structure. Since this highly oriented optically anisotropic pitch material formed from a fraction of an isotropic carbonaceous pitch has substantial solubility in pyridine and quinoline, it has been named neomesophase to distinguish it from the pyridine and quinoline insoluble liquid crystal materials long since known and referred to in the prior art as mesophase. Additionally, this separable fraction of the carbonaceous pitch capable of being converted to neomesophase is referred to as a neomesophase former fraction, or NMF fraction. Basically, the neomesophase former fraction of the pitch is isolated by solvent extraction of well-known, commercially available graphitizable pitches such as Ashland 240 and Ashland 260 to mention a few. The amount of neomesophase former fraction of the pitch that is separable, however, is relatively low. For example, with Ashland 240 no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to neomesophase.